1. Field of the Invention
The present invention relates to an apparatus and method for enhancing the performance of particulate collection devices. In particular, the present invention relates to the use of ammonia injection apparatus and methods that optimize the ammonia injection to increase the efficiency of particulate removal from waste gases.
2. Description of the Prior Art
Apparatus and methods for the removal of particulate from waste gases is known to be desirable. For example, in the combustion of fuels, in particular coal, various undesirable materials are released into the flue gas and are thereby released to the atmosphere. Among these are sulfur dioxide, sulfur trioxide, carbon monoxide, nitrogen oxides, and particulates. Particulate emissions are increasingly subject to legal limitations in terms of pounds per million Btu input, pounds per unit time, and in terms of the opacity of the Stack effluent. To meet these limits, and at times to reduce emissions for other reasons, various particulate collection devices have been employed by the operators of the combustion devices. Electrostatic precipitators, baghouses, cyclones, scrubbers, mechanical collectors, filter beds, electrified filter beds and other devices have been used to remove fly ash from the flue gases.
The most common particulate removal device employed by operators of the largest boilers is an electrostatic precipitator. U.S. Pat. Nos. 5,029,535 and 5,122,162 to Krigimont, et al. disclose a precipitator. Electrostatic Precipitation, by Oglesby and Nichols, Marcel Dekker, Inc., New York 1978, provides a good background on the subject. See also U.S. Pat. No. 3,523,407 to Humberg, and the Journal of the Air Pollution Control Association, Volume 18, No. 8, "Experience With Electrostatic Fly-Ash Collection Equipment Serving Stem-Electric Generating Plants," pp. 523, et seq., Reese and Greco, August 1968.
An electrostatic precipitator typically operates with a great number of wires which are charged negatively to as high as 60,000 volts and a number of grounded plates which are, of course, neutral. The particulate-laden gas passes horizontally between parallel plates which may be 9 inches apart, 20 feet high and 20 feet long. A field may consist of up to 40 of such parallel plates with as many as five fields in series in a precipitator. Between each adjacent plate are many wires. Each plate is grounded and each wire is charged negatively. The wires in each field may all be charged from a single source or the fields may be "sectionalized" in which each of two or more bus sections per field are energized by a separate source. Electrons are emitted from the wires and they ionize the gas immediately surrounding the wires developing a corona discharge, which is a rapid flow of ions. In the electrostatic field between the highly negatively charged wires and the grounded plates, the negatively charged ions are forced toward the grounded plates. Along the way, many of them collide with and become attached to the ash particles suspended in the gas stream. Then the particles become charged and, under the influence of the electrical field, migrate toward the collection plates. The magnitude of the force causing the particles to move toward the plates is proportional to the field strength and the charge on the particles. These particles arrive at the collection plate and are held there by a combination of mechanical, electrical, and molecular forces. The collected particles are removed by rapping the collection plates on a periodic basis. A thick layer of particulate matter must be collected so that it falls into the hopper as large agglomerates, so as to prevent excessive re-entrainment of the material into the gas stream.
The electrical field in an electrostatic precipitator exists from the wire to the plates. It exists through the collected ash layer. The average field strength is the voltage divided by the distance from wire to plate. If the potential is 60,000 volts and the wire to plate distance is 4.5 inches (11.43 cm) the average field strength is 5250 volts/cm. If the fly ash resistivity is too high the field strength is much greater in the accumulated fly ash layer. If the voltage gradient exceeds 15-20,000 volts/cm the fly ash break down and an arc occurs between the plate and the wire at the breakdown point. Current flows through the ionized gas at this point and no useful power is supplied to the bus section connected to the offending wire. This bus section is then automatically shut down and automatically restarred at a lower, but gradually increasing, voltage. This process retards the cleaning action of the precipitator and more fly ash goes out the stack with the flue gas. The fly ash is essentially glassy spheres containing the ash constituents of the coal mixed with unburned carbon. The chemical compositions of the glass spheres vary widely, depending on the source of the coal.
Fly ash is not very conductive. It may typically have resistivities as high as 10.sup.13 ohm-cm at the temperature where it is most resistive as shown in FIG. 1. The resistivity is lowered if the temperature is increased because the glass spheres are semi-conductors and are, therefore, more conductive at higher temperatures. Some electrostatic precipitators which were designed to operate on low sulfur western subbituminous coals were designed to take advantage of this fact by operating at 450 degrees F. or higher.
Many precipitators operate at temperatures below the temperature corresponding to the maximum resistivity. The lowering of the resistivity at the lower temperatures is naturally caused by condensation of sulfuric acid on the fly ash surface. The sulfuric acid is formed from sulfur trioxide and water vapor in the flue gas. At typical temperatures of 240 to 350 degrees F., the fly ash can usually be conditioned when there is 3 to 30 ppm of sulfur trioxide in the flue gas. The sulfur in coal is substantially converted to sulfur dioxide during Combustion. However, 0.2% to 2% of the sulfur in fuel is converted to sulfur trioxide in the furnace. This is the natural source of the conditioning agent that improves precipitation performance at 250 to 350 degrees F.
Many boiler operators are converting to lower sulfur coals in order to reduce sulfur dioxide emissions. If a boiler burns coal that has a sulfur dioxide emission potential of 1.2 lbs. sulfur dioxide per million BTU's, the flue gas will have about 600 ppm of sulfur dioxide if the gas is only slightly diluted with air and, at a conversion fraction of 1%, there would be only 6 ppm of sulfur trioxide in the flue gas. This is typically not enough to condition the fly ash and make it sufficiently conductive for good electrostatic precipitator performance unless the temperatures are extremely low, as a large fraction of the sulfur trioxide produced reacts with active sodium and calcium ions, making that fraction unavailable for fly ash conditioning. Many boiler operators have encountered this problem when switching to low sulfur coals.
Some operators have used sulfur trioxide conditioning to abate this problem. They add sulfur trioxide in the flue gas in amounts of 3 to 30 ppm in order to reduce the resistivity of the fly ash and improve the precipitator performance. This is an expensive process requiring corrosion resistant materials, multiple introduction points, a catalytic system to convert sulfur dioxide to sulfur trioxide, and storage and handling of liquid sulfur dioxide or burning of sulfur. In addition, when the boiler is operating at part load, the sulfur trioxide concentration may become too high and the resulting sulfuric acid can corrode the back end of the steam generator system. In addition some fly ash has a surface that is not easily coated by sulfuric acid and the problem of highly resistive fly ash can occur in spite of seemingly adequate levels of sulfur trioxide, either from high sulfur coals or conditioning systems.
Some operators have used ammonia conditioning systems. U.S. Pat. No. 4,064,219 to Yamashita, et al. discloses such a system as does U.S. Pat. No. 5,034,030 to Miller. See also The Journal of Air Pollution Control Association, Vol. 25, No. 2, "Conditioning of Fly-Ash With Ammonia," p. 152, Dismukes, February 1975. The success has been somewhat limited. Charles Gallaer in his Electrostatic Precipitator Reference Manual, EPRI CS-2809 published by Electric Power Research Institute, 1983, states
"Ammonia, NH3, has also been used to condition cold-side precipitators with uneven results. Although it cannot be shown to significantly reduce the resistivity of high resistivity fly ash, its use has helped some precipitators with this problem. It has been spectacularly successful in increasing the efficiency of precipitators handling fly ash of such low resistivity that they were "power hogs." In this latter application, it is presumed to react with the excess SO3 to form ammonium sulfate. This reaction not only increases the resistivity of the fly ash, but produces a fume having a large surface area. This fume, through its space charge effect, causes the precipitator to operate at a higher voltage and, therefore, to have a better performance. PA1 However, the reason why ammonia sometimes does and sometimes does not alleviate cold-side resistivity problems is still not completely understood".
Thus, it is seen that electrostatic precipitators sometimes do not operate well due to the high resistivity of the fly ash, that this problem is exacerbated by the continuing switch to low sulfur coal, that fly ash sometimes has a surface that retards the desirable action of the sulfuric acid, that sulfur trioxide conditioning is expensive, difficult and may damage parts of the combustion device, and that ammonia addition, as now practiced, is not always effective and sometimes actually increases fly ash resistivity which, while it may be desirable in the "power hog" case, is counterproductive when the problem is the more common highly resistive fly ash problem.
For the most recent few years baghouses have generally been the fly ash collection device of choice when a new unit is being designed to burn coal which will produce highly resistive fly ash. However, some baghouses operating on very resistive fly ash have not performed well. Too much of the fly ash passes directly through the bags. Also, with high resistivity ashes, it is difficult to clean the bags. The high resistivity ash, once it picks up an electrostatic charge, stays charged for long periods of time and firmly sticks to the bag by electrostatic and/or other forces. Cyclones, another particulate collection device, can be used as a scalping device to remove larger particulates prior to treatment of the flue gas by electrostatic precipitators or baghouses. Cyclones are relatively insensitive to resistivity of particulates, but are very sensitive to particle size.